(a) Field of the Invention
The present invention relates to a switching mode power supply (SMPS). More specifically, the present invention relates to an SMPS with a constant power control circuit for (a) controlling a current and voltage at a secondary coil of a transformer and (b) performing control to maintain a constant power at a predetermined interval such that a system employing the SMPS stably operates.
(b) Description of the Related Art
FIG. 1 is a schematic diagram of a conventional constant power control circuit. The conventional constant power control circuit comprises a rectifier 1, a constant power detector 11, a transformer 2, a switching unit 3 including an NMOS transistor, and a switching control unit 4 that is a pulse duration modulation controlling IC. The rectifier 1 comprises a capacitor C1 that smoothes the rectified voltage Vin. The constant power detector 11 comprises a resistor R1 having a terminal coupled to the capacitor C1 in parallel, and a resistor R2 coupled to the resistor R1 in series. The resistors R1 and R2 are coupled to a reference voltage terminal of the switching control unit 4.
The constant power detector 11 detects the voltage that flows to a primary coil through the rectifier 1 and provides the voltage to the switching control unit 4. In response to switching signals of the switching control unit 4, the switching unit 3 is turned on or off and provides to the switching control unit 4 the current that is detected when the switching unit 3 is turned on.
Therefore, the switching control unit 4 detects the current flowing at the primary coil using the voltage supplied from the constant power detector 11, and controls the level of the power that is supplied to the secondary coil using the current provided through the switching unit 3. That is, the switching control unit 4 uses the output voltage of the constant power detector 11 and the output current of the switching unit 3 so as to control the duty ratio of the switching signals that determines whether the switching unit 3 is controlled to an on or off state. Therefore, the switching control unit 4 ultimately controls the level of the power that is supplied to the secondary coil of the transformer 2.
Hence, the conventional art controls a maximum power, that is transferred from the primary coil to the secondary coil according to varied voltage of the primary coil, to maintain a constant value so that the maximum power supplied to the secondary coil is limited.
FIGS. 2 (a) and (b) show variations of the current and voltage of the conventional constant power control circuit of the SMPS. Referring to FIG. 2(a), a line a indicates the voltage of a battery, and a line b indicates the current provided to the battery. In an interval between A and B, a predetermined amount of the current is provided to the battery so that the voltage of the battery gradually increases, and in an interval between B and C, the current provided to the battery is controlled so that a rate of increase in the voltage of the battery is reduced. At this time, the power is controlled to maintain a constant value, and this power control is performed until the voltage of the battery reaches point C. In an interval between C and D, the voltage of the battery maintains a constant value, and the current is no longer provided to the battery.
Features shown in FIG. 2(a) are illustrated in a different format in FIG. 2(b) to explain the reason why power is controlled to maintain a constant value during a predetermined interval when charging a battery.
Referring to FIG. 2(b), in the interval between A and B, the current is provided to the battery by an amount equal to A so that the voltage of the battery reaches B. In the interval between B and C, the power is controlled to have a constant value. In the interval C and D, the voltage is maintained at a constant level and the current is reduced.
When the current having an amount equal to A continues to be provided to the battery, the voltage of the battery reaches point E. Since the voltage at point E is greater than the maximum power, the battery is damaged. Therefore, the power of the battery must be maintained at a constant value in the interval between B and C.
However, since the conventional art controls the power of the secondary coil according to the power of the primary coil, there occurs a reduction in the precision with which the power of the secondary coil is controlled. Accordingly, a power exceeding the maximum power can occur to damage the system. Furthermore, a voltage detector that comprises wattage resistors R1 and R2 detecting the voltage variations of the primary coil must be further installed to control the constant power.